Hi thanks for the fast reply. I tried your workaround but the flow-check tells me I have 2 errors.
1) Adjust invalid input for "Apply to each"
2) Adjust input parameters for "append to string variable"

Sadly my company doesn't allow a language switch so here is a translation for some terms:
"Zeichenfolge" = "String", "Auf alle anwenden" = "Apply to each", "@mention-Token für einen Benutzer abrufen"="Get @mention token for user", "An Zeichenfolge anfügen" = "Append to string variable"